Merge pull request mattn#462 from faruzzy/master

Updated "context" import since it has become a standard library

Merge pull request mattn#489 from Projectplace/fix-cancel-race

Fix race in ExecContext

Merge pull request mattn#479 from kenshaw/move-registeraggregator

Move RegisterAggregator implementation

fix test

upgrade amalgamation code

Merge pull request mattn#235 from jfrazelle/static_remove_dlopen

remove dlopen if static_build flag

update README.md